The present invention relates to an optical filtering device which blocks light to a desired region from within incident light while it transmits the remaining light therethrough, and a method of and an apparatus for comparing an optical image of an inspection object article and a reference image with each other using the optical filtering device and detecting a fine pattern defect or a foreign article from the difference between the images. More particularly, the present invention relates to a defect inspection method for carrying an appearance inspection of a semiconductor wafer, a photo mask, a liquid crystal device and so forth and an apparatus for the defect inspection method.
In fabrication of semiconductor devices, a substrate (wafer) on which a semiconductor device is to be formed is processed by up to several hundred fabrication steps to obtain a product. At the steps, a foreign article sticks to a substrate (wafer) or a pattern defect is caused by step dispersion in pattern formation or the like, and they make a semiconductor device defective. Further, in a defect inspection system for a semiconductor device, as miniaturization of patterns progresses, it is demanded not only to detect a finer defect or foreign article but also to detect an interesting object (DOI (Defect of Interest)). Simultaneously, needs for classification of various kinds of DOIs or defects which are not desired to detect are growing. To satisfy such needs, a defect inspection apparatus has been and is being developed, fabricated and sold in recent years which includes a plurality of detection optical systems and image processing systems (hereinafter referred to as detection head) and uses detection signals of the detection optical systems to achieve increase of the types of defects which can be detected and improvement in defect detection performance. Such a defect inspection apparatus as described above has been and is being applied to a semiconductor production line.
The defect detection apparatus for a semiconductor device is used to detect a pattern defect or a foreign article which occurs at such a step as, for example, a lithography step, a film-forming step or an etching step by inspecting the surface of a substrate after completion of the step and issue a cleaning carrying out instruction for the apparatus for the step. Or, the defect detection apparatus is used to detect, at an early stage, occurrence of a defective article by feeding a substrate, which is in a state in which it already suffers from a fatal defect, to succeeding steps.
A substrate for which a predetermined process has been carried out at a preceding step and on which a semiconductor device is being formed is loaded into an inspection apparatus. An image of the surface of the substrate (wafer) on which a semiconductor device is being formed is picked up and acquired, and such a defect signal decision threshold value defect decision process as disclosed in JP-A-2003-83907 (Patent Document 1), JP-A-2003-98113 (Patent Document 2), JP-A-2003-271927 (Patent Document 3) or the like is carried out based on the image thereby to carry out a defect decision. Then, the number of defects on the substrate and so forth are outputted.
If the detected defect number Nt is smaller than a defect number threshold value Nc set in advance, then the substrate (wafer) is sent as it is to a next step. If the defect number Nt is greater than the defect number threshold value Nc, then a cleaning carrying out instruction of the preceding step apparatus is issued, whereafter propriety of regeneration of the substrate is decided. If it is decided that the substrate can be regenerated, then the substrate is cleaned at a cleaning step and then is sent to the next step through the inspection step again.
In the substrate (wafer) which is an inspection object article and on which a semiconductor device is being formed, portions 1 and 1′ (hereinafter referred to each as die) having a same pattern are juxtaposed regularly as shown in FIG. 4. The defect inspection method and the defect inspection apparatus to which the present invention is directed compare images at positions in adjacent dies which have same inter-die coordinates with each other and carry out defect detection decision based on a difference between the images.
The semiconductor defect inspection system is ready for detection of a finer DOI and a requirement for a high speed inspection by adopting the following technique in addition to the technique described above. In particular, diffracted light from patterns on a semiconductor device is blocked using a spatial filter so that it may not be reflected on the inspection image to detect a foreign article or a defect on the semiconductor device with a high sensitivity as disclosed, for example, in JP-A-2000-105203 (Patent Document 4).
However, with such a spatial filter configured by juxtaposition of bar-like plates as disclosed in the above-mentioned patent document, it is difficult to block, for example, diffracted light of a plurality of pitches formed on the Fourier transform plane and caused by the presence of a plurality of pitches of patterns formed on a semiconductor device. Further, if a light source having a plurality of illumination wavelengths or a plurality of laser light sources of different oscillation wavelengths are used to illuminate a semiconductor device, then even if the patterns have a single pitch, diffracted light of a plurality of pitches is generated on the Fourier transform plane and it is difficult to block such diffracted light. Further, even if such light blocking is possible, the light blocking region becomes excessively great and this results in substantial reduction in size of apertures. This gives rise to a problem that the defect detection sensitivity drops.
On the other hand, if a light source having a plurality of illumination waveforms or a plurality of laser light sources having different oscillation wavelengths are used to illuminate a semiconductor device, then even if the patterns have a single pitch, it is similarly difficult to block light except a case in which the ratio of the plural light wavelengths and the pattern pitch on a semiconductor device have a special relationship. Or, even if it is possible to block light, the defect detection sensitivity drops from a similar reason.
From such reasons as described above, a defect inspection method which uses not a spatial filter wherein bar-like plates are arrayed one-dimensionally but a spatial filter having devices arrayed two-dimensionally has been proposed.
For example, as a two-dimensional optical filtering technique which can block light to an arbitrary region, a PLZT filter and a liquid crystal filter which use double refraction are available. However, since the former has a wavelength dependency in working characteristic thereof, filtering of illumination light of a plurality of wavelengths is difficult. The latter can detect only scattered light of particular polarized light, and scattered light components polarized in a direction perpendicular to the polarization direction are blocked. Therefore, depending upon the polarization characteristic of scattered light from a defect, the detection sensitivity drops significantly. Further, the latter has such a subject that the durability when ultraviolet rays are irradiated thereon is low and gives rise to a problem that the light source to be used for inspection is limited.
Therefore, development wherein a device in the form of a two-dimensional array which does not utilize a double refraction effect is utilized as a spatial filter is being progressed. Among such devices, a device draws attention which uses a MEMS (Micro Electro Mechanical System) technology because the ratio in transmission light intensity upon ON/OFF switching (light transmission ratio) thereof is high.
As a device for separating an arbitrary region in which the MEMS technology is used and any other region, a method which uses such a two-dimensional DMD (Digital Micromirror Device) as disclosed in JP-A-2004-170111 (Patent Document 5), another method which uses such a micro actuator array and micromirrors as disclosed in JP-A-2004-184564 (Patent Document 6) and a further method which uses an array of micro shutters by T. Takahashi et al. are available.
Of the methods mentioned, the former two methods, namely, the methods which use micromirrors, utilize that, if a desired potential difference is applied to different portions of a device to change the direction of the mirrors, then the direction in which light irradiated toward the mirrors is reflected changes.